The operation in which the mole -- who travelled on a European Union passport that would have gotten him through U.S. security, should that have been a part of the plot -- had been in place for several months and the American role in the operation only took on operational urgency in the past two or so months, the officials said.

The long running operation with the deep cover operative was one that intelligence agencies planned to keep running. It was pulled up short in the past week when leaks developed and put the infiltrator in jeopardy. Sources involved in the intelligence operation said the plan was to keep the operation running until a more complete picture of the still developing plots and plans of the Yemen based group and its sinister, creative bombmaker, were learned.

"This was gold dust," one senior intelligence official said. "Such assets are few and far between."

Authorities would not discuss the whereabouts of the wanted bombmaker, known as Ibrahim al-Asiri, at this time.